The present invention relates to an exhausting device, and more particularly to one for a two-stroke engine.
The scavenging process of a two-stroke carburreted gasoline engine is accomplished by using the unburn air fuel mixture to scavenge the residual burnt gas off the cylinder but suffers from the following disadvantages:
1) The new mixture coming from the scavenging ports is partly discharged out of the exhaust port before the closing of the exhaust port; which is so called as the "short-circuitting loss of unburn mixture". Under a low load, the intake new mixture is relatively less and can almost be kept in the cylinder so that the "short-circuitting lose of unburn mixture". Under a low load, the intake new mixture is relatively less and can almost be kept in the cylinder so that the "short-circuitting loss" is relatively small. When the engine has an increasing load and the intake air or mixture is in turn getting more and more, the "short-circuitting loss" is accordingly increasing. As shown in FIGS. 2A and 2B showing a two-stroke engine subjected to a relatively high load, when the piston 37 reached the bottom dead center and begins to upwardly move (as shown in FIG. 2A), the scavenging mixture front 31 continues to flow toward the exhaust port 16 and the upwardly moving piston 37 will compress the cylinder gas which results in that before port 16 is closed by piston 37, some new mixture, which is the source of the short-circuitting loss and will be more if the engine load is larger, has been discharged out of port 16 as schematically shown in FIG. 2B.
2) The new mixture flow cannot effectively scavenge the residual gas off the cylinder; which becomes worse when the engine has a low load or an idle speed since the intake new mixture is relatively less as schematically shown in FIGS. 3A and 3B. When piston 37 reached the bottom dead center and begins to upwardly move as shown in FIG. 3A, the scavenging air front 41 may not reach port 16 since the intake new air flow from the scavenging port 11 is relatively weak. Even after port 16 is closed by piston 37 as shown in FIG. 3B, scavenging air front 41 still may not reach the opposite cylinder wall surface 17 so that the cylinder inevitably contains therein the residual exhaust gas 42 resulted by the last combustion cycle which leads to that the probability of igniting the mixture is reduced, the probability of the misfire of the mixture is increased and the flame propagation speed is also reduced all of which are true for the loop-scavenging type, the cross-scavenging type or the mixed type as respectively shown in FIGS. 1A, 1B and 1C. As an example, the exhaust gas and the new air can have a volume ratio of 55:45 or even higher volume ratio under an idle operation of the engine which is the primary reason that the engine has a low combustion efficiency.